The expression “on-line” generation of data (or performance of another operation) during a reaction herein denotes generation of the data (or performance of the other operation) sufficiently rapidly so that the data (or result of the operation) is available essentially instantaneously or sometime thereafter for use during the reaction. The expression “generation of data in on-line fashion” during a reaction is used synonymously with the expression on-line generation of data during a reaction. Generation of data from a laboratory test (on at least one substance employed or generated in the reaction) is not considered “on-line generation” of data during the reaction, if the laboratory test consumes so much time that parameters of the reaction may change significantly during the time required to conduct the test. It is contemplated that on-line generation of data may include the use of a previously generated database that may have been generated in any of a variety of ways including time-consuming laboratory tests.
With reference to a product being produced by a continuous reaction, the expression “instantaneous” value of a property of the product herein denotes the value of the property of the most recently produced quantity of the product. Because of the back-mixed nature of a gas phase polymerization reactor, the most recently produced polymer product typically undergoes mixing with previously produced quantities of product before a mixture of the recently and previously produced product exits the reactor. In contrast, with reference to a product being produced by a continuous reaction, the expression “average” (or “bed average”) value (at a time “T”) of a property herein denotes the value of the property of the product that exits the reactor at time T.
The expression “polyethylene” denotes at least one polymer of ethylene and optionally one or more C3-C10 α-olefins, while the expression polyolefin denotes at least one polymer (or copolymer) of one or more C2-C10 α-olefins.
Throughout this disclosure, the abbreviation “MI” denotes melt index. Also throughout this disclosure, the term “density” denotes the intrinsic material density of a polymer product (in units of g/cc unless otherwise stated), measured in accordance with ASTM-D-1505-98 unless otherwise stated.
One method for producing polymers is gas phase polymerization. A conventional gas phase fluidized bed reactor commonly employs a fluidized dense-phase bed typically including a mixture of reaction gas, polymer (resin) particles, catalyst, and (optionally) other additives. Typically, any of several process control variables will cause the reaction product to have certain, preferably desired, characteristics.
Generally in a gas-phase fluidized bed process for producing polymers from monomers, a gaseous stream containing one or more monomers is continuously passed through a fluidized bed under reactive conditions in the presence of an activated catalyst. This gaseous stream is optionally withdrawn from the top of the fluidized bed and recycled back into the reactor. Simultaneously, polymer product is withdrawn from the reactor and new monomer is added to replace the polymerized monomer. The recycled gas stream is heated in the reactor by the heat of polymerization. This heat is typically removed in another part of the cycle by a cooling system external to the reactor.
It is important to remove heat generated by the reaction in order to maintain the temperature of the resin and gaseous stream inside the reactor at a temperature below the polymer melting point and/or catalyst deactivation temperature. Further, heat removal is important to control the reactor temperature to prevent excessive stickiness of polymer particles that if left unchecked, may result in loss of fluidization or agglomeration of the sticky particles which may lead to formation of chunks or sheets of fused polymer that cannot be removed as product.
Conventional fluid bed polymerization reactors often pretreated (e.g., by “chromocene” treatment) to form a polymer film on the reactor wall surfaces that will be exposed to polymer resin during normal polymerization operation. Such a polymer film coating on the bed wall of a pretreated reactor is intended to function as an insulating layer that reduces static charging in the reactor system, thereby reducing the potential for sheeting, during normal polymerization reactions. It is believed that static charging of polymer (e.g., polyethylene) resin in the bed during polymerization is strongly influenced by the electrical interaction between the polymer wall film and the reactor/cycle gas, and is thus strongly influenced by the electrical characteristics of the polymer wall film. For example, a thick insulating wall film would limit charge transfer from the polymer in the bed to ground.
Although the polymer wall film on a precoated reactor bed wall is typically thin (e.g., in a range from about 1 to about 20 mils, or 0.025 to 0.50 millimeters, where one “mil” denotes 0.001 inches) and typically does not have uniform thickness throughout the bed wall, it can be effective in reducing static charging and is often durable. Often, a typically thin polymer film of this type has a service life of at least four years before retreatment is required, if (as is typical) the film consists of a high density, high molecular weight (very low melt index) polymer. Such a film having high density, high molecular weight, and low melt index, is typically highly resistant to abrasion by the softer polymer typically present in the fluid bed during normal polymerization operation.
Conventional chromocene treatment methods can form effective and reliable polymer coatings on the bed walls of fluidized bed polymerization reactors. Sometimes, however, such methods fail to form effective and reliable polymer coatings and instead form insufficiently thick polymer on at least some bed wall portions. Without an effective polymer coating, a reactor that has undergone such failed treatment is sensitive to static charging and sheeting, particularly during polymerization reactions using metallocene catalysts.
Also, the polymer coatings formed by conventional chromocene treatment methods on bed walls (of fluid bed polymerization reactors) can deteriorate or become contaminated over time. For example, they can deteriorate as a result of erosion and/or deposition of impurities thereon (e.g., decomposition products of aluminum alkyls). Such deterioration and/or contamination can have a major effect on operability of the reactor. In practice, the reactor static baseline does not change suddenly due to the contamination or deterioration. Rather, the contamination or deterioration usually occurs over a period of time and as this happens, static activity and sheeting problems gradually develop and appear first during the production of certain resin products. These products, usually characterized as having higher molecular weights and higher densities, are referred to as the sensitive reactor grades. As the static baseline deteriorates further (e.g., as the wall coating becomes more contaminated) static and sheeting problems begin to occur with more and more products. The sensitivity of sheeting risk to different resin grades appears only with a contaminated or deteriorated bed wall coating. It is desirable to operate the reactor with the coating in good condition.
It is conventional to perform reactor system retreatment to remove a bad (deteriorated or contaminated) bed wall coating and replace it with a new polymer coating when necessary. Conventional retreatment methods involve preparation of the bed wall (typically by removal of an existing bad polymer coating) and the in situ creation of a new polymer coating on the wall. One such conventional retreatment method is the above-mentioned chromocene treatment method; another is known as hydroblasting. Wall retreatment is expensive and requires reactor shutdown for retreatment. It would be desirable to have a reliable method for monitoring the state of an existing bed wall coating during polymerization operation of a reactor, e.g., to determine when retreatment is or is not needed.
In the past, polymer coatings on the bed walls of fluidized bed polymerization reactors were typically inspected on an opportunistic basis (when the reactors were shut down) by persons who physically entered the reactor vessels with appropriate inspection instruments. Alternatively, the conventional metal coupon approach was used to inspect the coatings but this technique had to be performed in an off-line fashion under conditions not necessarily representative of actual operating conditions. There is a need for a method for monitoring (e.g., inspecting and/or characterizing) polymer coatings on bed walls of fluid bed polymerization reactors (e.g., to assess whether the coatings have deteriorated or become contaminated) during performance of polymerization reactions in the reactors (e.g., in on-line fashion during each reaction using a probe external to the reactor).
Herein, the expression that a probe is “external” to a reactor (or is an “external” probe) denotes that the probe is configured and mounted so as neither to interfere significantly with nor significantly affect a polymerization reaction occurring in the reactor during operation of the probe to monitor the reaction or reactor. For example, a probe having a distal portion (e.g., tip) that is flush with a bed wall of a reactor or extends slightly into the bed from the bed wall may be an “external” probe if the probe neither interferes significantly with nor otherwise significantly affects a polymerization reaction in the reactor while the probe operates during the reaction to monitor voltage in the bed (or to generate bed voltage data that is used to monitor a film that coats the bed wall and the probe's distal portion).
Herein, “bed static” denotes static charge (and/or the electrical potential due to such charge) that is generated by frictional contact involving contents of a fluid bed polymerization reactor (e.g., polymer resin). For example, bed static can result from frictional contact of polymer resin in the bed with the reactor's bed wall (the wall in the fluidized bed section of the reactor). The bed wall can be coated or uncoated. It is conventional to monitor reactor bed static using probes external to the reactor. Static probes suitable for measuring bed static are described in, for example, U.S. Pat. Nos. 6,008,662 and 6,905,654.
However, it had not been known how to use the output of one or more static probes (e.g., data indicative of voltage and/or current readings, generated using an external static probe) to monitor a property of a coating (e.g., polymer coating) on a bed wall of a fluid bed polymerization reactor during performance of a polymerization reaction in the reactor (or to monitor a property of the bed wall itself during performance of the reaction in the reactor). Nor had it been known how to use the output of one or more static probes (e.g., external static probes) to assess whether such a coating (or the bed wall itself) has deteriorated or become contaminated.
In a class of embodiments, at least one static probe (e.g., an external static probe) is used to monitor at least one property of a coating (typically a polymer coating) on a bed wall of a fluid bed polymerization reactor (e.g., to assess whether the coating has deteriorated or become contaminated) during performance of a polymerization reaction in the reactor. In some embodiments, the monitoring is performed in on-line fashion during the reaction. In some embodiments the bed wall is not coated and at least one static probe is used to monitor at least one property of the wall itself (e.g., to assess whether the wall has deteriorated or become contaminated) during performance of the reaction.
A shortcoming of conventional static probes (for monitoring fluid bed polymerization reactions) is that they measure only one of the following two entirely different effects: (i) current flow by direct contact with the bed (i.e., charge transfer from the bed to a probe surface); and (ii) inductive current flow (in which the electric field inside the reactor induces a charge on the probe surface without direct contact with the probe). It would be desirable to measure both these effects simultaneously at the same location in the bed.